RF power amplifiers (PAs) are typically used for propagating amplitude and phase information for a RF input signal in an accurate manner so as not to corrupt data being transmitted within the RF signal. Generally, in order to attain this accuracy, a large bias current is used to bias the PA at an operating point with sufficient gain and bandwidth. Unfortunately, in battery-operated equipment, this large bias current is not conducive to long battery life, it is also not conducive to optimum linearity and to low noise operation.
Low noise amplifiers (LNA) are typically biased at a fixed current for minimum noise operation so that small amplitude input signals are detectable. In determining the bias point for an active device performing signal amplification, there is a trade-off between noise figure (Fmin) expressed as dB and other amplifier figures of merit. However, where the active LNA device is in close proximity to the transmitter, for example as in an antenna-coupled LNA, situations may arise where the input signal provided to the input port of the LNA causes overload because it has a high signal power. LNAs typically have low gain, but, often do not have the low noise performance when biased for best linearity. Conversely, they often do not have best linearity when biased for lowest noise. PAs on the other hand, typically have low gain when deliberately biased at low currents. It is known to those of skill in the art of PA design, that PAs are sometimes deliberately biased at low currents and rely on an autobias effect from the signal to achieve overall linearity. That is, the magnitude of the input signal to the amplifier actually influences the bias point of the amplifying transistor so as to improve linearity. Stronger input signals increase the bias current of the amplifying transistor. This effect, however, is limited and there exists a need to provide a means to enhance the autobias effect and improve linearity with even stronger input signals.
It is an objective of this invention to augment the autobias effect by providing bias-boost circuitry that is effectively autobiased by the output signal of the amplifying transistor.